Object oriented approach to plot.ly

### Overview 
The session is stored in the state of the object and the Matlab plotting functions are implemented as overloaded methods.

### The syntax

    pl = plotly('olegkoma','mykey');
    plot(pl, usual matlab syntax for plot)

### Advantages 
Seamless integration of the plotly API. 

The signup is prompted on first call of plotly(), and credentials are saved at the end (also prompted). Successive calls of pl = plotly(), will automatically load credentials. 

Signup() and savecredentials() are also available externally to the object, so that you can create other profiles and overwrite existing saved credentials.

### Useful commits to main proj
Apart from specific object oriented implementations, the first 3 commits fix some path handling for windows systems, getuserdir() for win retrieves %APPDATA% (safer) and prompts are implemented for signup (for username, email, and for saving credentials).