Add "String to SymTerm" parser based on Parsec
Add parser for Ln
Parser: Ignore whitespace
Generic association lists. Not very useful yet.
Beta: evalTermP evaluates a term containing Variables

Using an association list
A bit of documentation and some trivial transformations
Added abs() clause
Merge branches 'pu/parse', 'pu/eval', 'pu/simplify' and 'pu/derivate' into next

Test release
Changed simplify's behavior

By introducing simplify as new function and renaming the old simplify
function into simplifyOnce we can avoid infinite recursion; simplifyOnce
calls the specific function for a term (e.g. simplifySum) which
transforms it by one step. simplify calls simplifyOnce as long as
simplifyOnce delivers a term different from the one before
(like `until simplifiedTerm == term; simplify (simplifyOnce term)`)

This commit introduces some new rules as well.
Beta: Factor out common factors in products

Simplify now transforms a term like
    b * a * c + b * c * d
to
    b * c * (a + d)

It's not fully functional yet.
Replaced (==) by elem

well, that's awkward...
Moved fromMb to Symmath.Util
Prototype of a simple function value table function
Beta: Function value table.

TODO:
    - Align the table as Perl would do it with its formats.
    - Allow rounding to more or less decimals than 3
Miscellaneous changes

- Fixed bug: x * (y * y) was transformed to the non-resolvable product
(x * y) * y without checking if (y * y) could have been simplified.
- New rule: x * x^2 => x^3
- Variables go before other terms when sorting products
Fixed wrong clause order in simplifyFrac
Unnecessary clause removed; some cosmetics
Added unit test for simplify

From the next release on, the testing "infrastructure" consisting of the
makefile and the template in TestSimplify.hs can/should be used to cover
all Symmath functions. Who wants to be Testing Engineer?
Fixed infinite recursion in some scenarios
Better clause structur in simplifySum. Fixed regression