Complete the ORM Function Library

[Matthew Pava]

I was surprised to learn that we didn't have a StrIndex function until version 2, and yet we had Substr since at least version 1.8. I wonder how users were using Substr without also finding a use for StrIndex this whole time. Anyway, since we seem to be adding these functions one at a time, why don't we work on trying to get the built-ins implemented in one sweep instead?

We may even want to split the documentation page (​https://docs.djangoproject.com/en/dev/ref/models/database-functions/) into further categories with String functions and Numeric functions.

This is just a sample checklist. There are probably several more that I have missed. These functions are available directly out of the box with PostgreSQL.

String functions not yet implemented:
ASCII(): Returns numeric value of left-most character
CHR(): Character with the given code.
LEFT(): Returns the leftmost number of characters as specified
LPAD(): Returns the string argument, left-padded with the specified string
LTRIM(): Removes leading spaces
MD5(): Calculates the MD5 hash of string, returning the result in hexadecimal
REPEAT(): Repeats a string the specified number of times
REPLACE(): Replaces occurrences of a specified string
REVERSE(): Reverse the characters in a string
RIGHT(): Returns the specified rightmost number of characters
RPAD(): Appends string the specified number of times
RTRIM(): Removes trailing spaces
TRIM(): Removes leading and trailing spaces

Numeric functions:
ABS(): Returns the absolute value.
ACOS(): Returns the arccosine.
ASIN(): Returns the arcsine.
ATAN(): Returns the arctangent.
ATAN2(): Returns the arctangent of the two variables passed to it.
CEILING(): Returns the smallest integer value that is not less than a numeric expression
COS(): Returns the cosine expressed in radians.
COT(): Returns the cotangent.
DEGREES(): Returns a numeric expression converted from radians to degrees.
EXP(): Returns the base of the natural logarithm (e) raised to the power of a numeric expression.
FLOOR(): Returns the largest integer value that is not greater than a numeric expression.
LOG(): Returns the natural logarithm of a numeric expression.
MOD(): Returns the remainder of one expression by diving by another expression.
PI(): Returns the value of pi
POWER(): Returns the value of one expression raised to the power of another expression
RADIANS(): Returns the value of an expression converted from degrees to radians.
ROUND(): Returns a numeric expression rounded to an integer. Can be used to round an expression to a number of decimal points
SIN(): Returns the sine given in radians.
SQRT(): Returns the square root.
TAN(): Returns the tangent expressed in radians.