Added auto-completion of function signatures

This PR adds the functionality to complete function signatures as you type '(' (open parantheses) after typing a method name. The following are example scenarios on how it looks like:
1. Auto-completion of global (built-in) methods
![image](https://cloud.githubusercontent.com/assets/5710686/4609197/a461e002-5298-11e4-884a-63ecba79cc83.png)

2. Completion of static methods and constructors
![image](https://cloud.githubusercontent.com/assets/5710686/4609198/a8def1ba-5298-11e4-8ca6-d9b6bdf3fbe1.png)

3. Completion of instance methods
![image](https://cloud.githubusercontent.com/assets/5710686/4609200/add2fe1e-5298-11e4-9aa1-b40a21777760.png)

Future Enhancements:
1. Highlighting parameters in function signature tooltip is not supported yet and will be handled in successive implementations
2. Function signature completion from a selection listbox as per EB's designs: https://docs.google.com/a/adsk-oss.com/document/d/1gzgyVMbpK_r7OFx2IoDXF2J5lQgTdOUA-SQ7_WB1nZ8/edit

@Benglin please take a look. Thanks.