Adding a bunch of mathy functions to cypher.

Other databases have these in their SQL. I think Cypher should have them... most of them are pretty trivial.

```
ACOS(number)
ASIN(number)
ATAN(number)
ATAN2(y, x)
CEIL(number)
COS(radians)
COT(radians)
DEGREES(radians)
E() = math.E
EXP(number)
FLOOR(number)
LOG(number)
LOG10(number)
PI() = math.Pi
RADIANS(degrees)
RAND() = math.random (double from 0.0 to 1.0)
SIN(radians)
TAN(radians)
```

The only potentially controversial one is rand() maybe? There were some requests for it, and it is pretty useful for order by, in particular, when you want a pseudo-random order. Anyway, the implementation has a caveat that causes rand() to be the same per result row, which might not be perfect, also. Andres?

Anyway, feedback welcome.