vec2 and mat2

[sinisterchipmunk]

Some operations to assist in 2D operations. I'm fairly new to 2D so hopefully this is of use. I'll understand if this pull request is rejected. :)

Adds the following, which function the same as their vec3/mat4 counterparts, as well as corresponding tests:

vec2.create
vec2.add
vec2.subtract
vec2.multiply
vec2.divide
vec2.scale
vec2.dist
vec2.set
vec2.negate
vec2.normalize
vec2.cross
vec2.length
vec2.dot
vec2.direction
vec2.lerp
vec2.str
mat2.create
mat2.set
mat2.identity
mat2.transpose
mat2.determinant
mat2.inverse
mat2.rotate
mat2.scale
mat2.str
mat2.multiply
mat2.multiplyVec2
mat3.multiplyVec2

[toji]

Looks great! The only one that I somewhat question is the vec2.cross, but you've been pretty thorough about explaining where the logic from that one comes from.

[sinisterchipmunk]

Thanks :) My first impulse was to have vec2.cross always return a vec3 but since two components are guaranteed to be zero, I thought having the option of receiving a scalar would be more useful from a performance perspective.