PLFormatter only enters a tab if beginning on an empty line.

This is a set up for the tab cycler, which will determine the appropriate place to tab. Empty lines receive a normal tab (replaced with whitespace).
Added interface for opening and saving files in the Text Editor View Controller.
The text editor view controller has an interface for opening and saving files. The code for opening files by opening an NSOpenPanel has been implemented.
Key equivalents are sent through the tab view controller to the view controller objects.
Text editor view opens NSOpenPanel on cmd+o event.
EditorViewController responds to cmd+o, cmd+shift+s.
Implemented the open and save as functionality for the editor view controller.
Text Editor saves when pressing cmd+s.
Added status text field indicating that a save occurred.
On save, the status text field in the status bar indicates that the file at the indicated url was saves.
Perform selector in editor view controller verifies the appropriate modifier keys were not pressed.
PLSyntaxHighlighter now highlights scientific notation.

Fixes #1.
PLFormatter didFormatAfterTab:withReplacementString:inRange now can return user to indentation level of previous line.

A specific example: if in a nested function definition, pressing tab will insert the number of whitespaces necessary to reach indentation level of the previous line.
PLFormatter prevents tabbing past the proper indentation level.
If after the proper indentation level, the PLFormatter now deletes the extra text, returning the insertion point to the proper indentation level.

For safety, this is only done if the entire line is whitespace. If there are any non-whitespace characters, pressing tab does nothing.
PLEditorViewController processes syntax highlighting in two steps now.

Step 1: get the edited line range in the textStorageWillProcessEditing: method. Save this to a private attribute of the PLEditorViewController class.
Step 2: color the line with syntax highlighting in the textStorageDidProcessEditing: method using the edited line range to determine where to do so.

Fixes #2.
Added line numbers from the start of a new editor.
Merged develop and feature-open_and_save to incorporate fixed issue 1 and 2 to feature branch
Moved key equivalent logic from tab subview controller to tab view controller.
Tab subview controllers must conform to the PLTabSubviewController protocol.
Cmd+w opens an alert window if the text editor has unsaved changes.
Interpreter view conforms with PLTabSubviewController protocol.
Save and open messages do not have effects: new feature may be to save and restore interpreter sessions.
PLEditorViewController now stores a history of the most recent two edits in a NSMutableArray.

PLFormatter methods have been updated to receive a history of edits from the NSTextView (sent from the PLEditorViewController).
Merge branch 'develop' of https://github.com/dnicklas/Liasis into feature-cycle_tabs

Conflicts:
	Liasis/Liasis/TextEditor/PLEditorViewController.h
Key equivalents for open, save and close use standard tab view controller methods.
The key equivalents for these methods no longer send messages directly to the tab subview controllers, but instead send a message to the tab view controller indicating that the active subview should open, save or close, respectively. These methods will also be linked with the menu bar open and save.