Fix inlining when using a name other than glslify

Previously, glslify was assuming the variable name used for
requiring would be "glslify". This would mean the following
would not work:

    var createShader = require('glslify')

    createShader({
        vertex: './file.vert'
      , fragment: './file.frag'
    })

And the following would still get replaced:

    glslify('no requires')

Using the sleuth module makes it easy to discover what
variable name is being used and use it with replace-method.

Have also included preemptive checks to bail before parsing
files which definitely don't contain calls to "glslify"
within.
style tweaks, remove first-match
Merge pull request #4 from chrisdickinson/fix-inlining

Fix inlining when using a name other than glslify