fix handling of sign text options

These options weren't used properly, because the s:line_* functions were
overwriting the sign texts. Now we initialize them only once at plugin
start.

Added Otto Modinos as contributor. Thanks for reporting.

Closes pull request #19.
use vim-style modeline
fix sign overwriting regex

Signs (from Syntastic in this case) were overwritten, although
g:signify_sign_overwrite was set to 0, because the used regex was buggy.

Add Ryan Kois as contributor.

Closes #20.
change version number -> 1.6
merged master