Change the dialog's title

Now the title for the dialog is 'Clusterpy light: Max-p'
This closes #12
Fix max-p dialog layout

This commit closes #13 and closes #14 both related to the max-p dialog
Clean after success

In this changes I clean the output layer field after a successful
execution, closes #16
This change also disables the OK button while the algorithm is running
but the Cancel button is still enabled, this is related to #6
Fix error message

Changed the message.
Changed the type of ID, now the str on each element will get rid of the
'L' for long. Making the message clearer. This closes #15

Quick reference to #16 because now the output and progress bar get
cleared even after an unsuccessful run.
Update the help and about sections

The about section now has:
 * Link to web, closes #18
 * Names of dev team, closes #17
 * How to cite the plugin, relates to #25
 * Reference to Clusterpy, closes #19
The maxphelp section now has:
 * Reference to Clusterpy, closes #19
Merge pull request #28 from sergiobuj/fix_ui

Fix ui branch