Unit tests

[robatron]

Adds unit tests. Fixes issue #1.

Notes:

• Modularized and documented helper functions
• Global tasks variable still maintains tasks registry, and is passed as a parameter and manipulated by reference by modules as necessary
• Three test specs in test/load-subfile-spec.js are left unimplemented due to implementation complexity
• Added lodash as a runtime dependency to standardize type validation
• Hand-verified that examples/gulpfile.js produces the same output as master

[robatron]

Just added unit test coverage reporting with istanbul. These unit tests get us to ~86% line coverage:

----------------------|-----------|-----------|-----------|-----------|
File                  |   % Stmts |% Branches |   % Funcs |   % Lines |
----------------------|-----------|-----------|-----------|-----------|
   lib/               |     86.11 |     87.04 |     77.78 |     85.85 |
      add-subtask.js  |       100 |        95 |       100 |       100 |
      add-task.js     |     88.89 |       100 |        75 |      87.5 |
      get-subfiles.js |       100 |       100 |       100 |       100 |
      hub-util.js     |       100 |       100 |       100 |       100 |
      index.js        |       100 |     85.71 |       100 |       100 |
      load-subfile.js |     39.13 |     54.55 |        25 |     39.13 |
      resolve-glob.js |       100 |       100 |       100 |       100 |
----------------------|-----------|-----------|-----------|-----------|
All files             |     86.11 |     87.04 |     77.78 |     85.85 |
----------------------|-----------|-----------|-----------|-----------|

=============================== Coverage summary ===============================
Statements   : 86.11% ( 93/108 )
Branches     : 87.04% ( 47/54 )
Functions    : 77.78% ( 14/18 )
Lines        : 85.85% ( 91/106 )

[robatron]

Ok, now we're up to ~87%

[frankwallis]

superb