Commented all files in the go implementation of Euclid's algorithm.

Used better variable names in euclid_test.gow
Used better variable names in euclid.go
Changed package for Java's implentation of Euclid's Algorithm from
main to euclid.
Changed euclid to return a 0 value if either input is 0 to stay
consistant with the Go implementation.

Better comments and variable names have been used.
Commented functions and used better variable names. Updated error
reporting in unit test to be more precise.