Some minor readability improvements
+ Added default constructor to MathOperationLimit
+ Added support for ln() in ModelHelper
- Removed the possibility for MathOperation to have null as a child

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixes #68
* Also some parentheses placement fixes

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Added the toString() to MathParentheses and MathOperationSubtract

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixed a bug where EvalHelper inverted the base and exponent of a root

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The state is now saved for FragmentMainScreen when rotating the screen.
This fixes #30

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Created convenience methods fromXML() in MathFactory

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The evaluation dialog now saves its state when switching between portrait and landscape mode
+ Also set a maximum width for the evaluation dialog on tablets

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixed a bug where an integral with empty from and to children couldn't be sent to Wolfram|Alpha

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The FragmentKeyboard state is now saved when switching between landscape and portrait mode

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Parts of the expression can now be removed from the expression

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Drawing the MathObject is now cached by MathView
Fixes #63

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Changed the default size of expressions to a better value for normal screens

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Parentheses centerFix
#69 refactor fix