Add expression registration facility

* Add registration facility for custom AbstractExpressions
* Rework SpecialExpression to ExampleExpression
* Add example documentation
Merge pull request #13 from bastih/feature/tablescan_improvements

Add ability to create custom expressions

* Add registration facility for custom AbstractExpressions
* Rework SpecialExpression to ExampleExpression
* Add example documentation
This tests a server start
Merge pull request #17 from bastih/feature/test_server_start

Finally test an actual start of the server and assure existence of both /query/ and /urls/.
Introduce new base class for storables
Add abstract resource type forward declaration
Radix Clustered Hash Join is now parallel.

The radix join adds the following plan operatoins:

- Histogram - calculates occurrences per value in a table
- Histogram 2nd pass
- Radix Cluster - clusters an input table based on a prefix sum list and
  a hash value
- Radix Cluster 2nd Pass
- Prefix Sum - generates a prefix sum array based on the histogram
- Merge Prefix Sum
- Barrier
- Create Radix Table

The script in tools/joiner.rb can be used to generate paralle
radix join plans. In addition there are a few changes to the
interface of the fixed length vector to better handle the
down-casted data access. This includes a inc() and
atomic_inc() method to increment a certain value at poin.

Limitations:
  -> Warning: Only Integer values can be joined
Merge pull request #15 from lordgeorg/master

Adds table name display to difference table test output
Adds String and Float Joining Capabilities

This commits adds the possibility to join string and float columns.

Closes #20
Changes the behavior for the PlanOperation::distribute() method

Now the out paramers will be start and end with end be one after
the acutal element so it can be used as in traditional while x < end
comparisons
Merge pull request #22 from hyrise/feature/table_partitioning

Changes the behavior for the PlanOperation::distribute() method
added parallelization for virtual radix join operator
Clean up HashTable constness and some related issues
Merge pull request #23 from bastih/feature/cleanup_hashtable_constness

Clean up HashTable constness and some related issues
Improve getAttributeVector non-implemented exception
changed hyrise server to not shut down with execption if hwloc_set_membind is not supported
two fixes on join parallelization - number of join ops is now restricted to min(join_par, partitions)
changed distribution of partitions to joins to distribute partitions more evenly if no of joins is small (similar to distribute function in PlanOp)
added workstealing task scheduler and set this scheduler as default
Extended RadixJoin to work with PointerCalculators - still needs testing and cleanup