[DBAL-139] [WIP] Finish CACHE option implementation for sequences

@beberlei Started implementation for [DBAL-139](http://www.doctrine-project.org/jira/browse/DBAL-139) in commit https://github.com/doctrine/dbal/commit/e8efcd7621e85003f3ce496bc26a108b53371838 and https://github.com/doctrine/dbal/commit/d397c005dd3ff3d48a81eeb47c2a0d74d6b7400d which was first drafted in PR #202. It's about adding a cache option for sequences.

**The following missing aspects of the implementation were added:**
- SQL Server support
- SQL Anywhere support
- Reverse engineering the cache option with the schema managers
- Comparator implementation
- Schema::createSequence() adjustment
- More tests

**Additional optimizations:**
- Rename `$cache` to `$cacheSize` in Sequence (unified naming with `$allocationSize`)
- More strict input validation for cache size values in Sequence to avoid SQL generation errors
- Better API documentation
- Unified `getSequenceCacheClause()` method on all platforms

**Todo:**
- Fix comparator issues (see below)
- Finish test suite for this (depends on comparator issues solution)

**Comparator issues**
The comparator will get into trouble when comparing cache size values. To get an understanding of this I'll first explain how cache size values evaluate to the database:

- `null` -> No cache size specified, don't generate cache clause for sequences and use platform's default
- `0` -> Explicitly disable caching for sequences, generates `NOCACHE` clause on platforms
- every other positive value is an explicite specification of the cache size and results in `CACHE <size>` clause

Now there are two problems.

First one is that a value of `1` has a special meaning on some platforms, too. Oracle does not accept it and throws an error as the minimum allowed value is `2`. PostgreSQL allows a value of `1` but in fact interprets it as `NOCACHE`. SQL Server and SQL Anywhere accept a value of `1` but the behaviour is unclear compared to an explicite `NOCACHE` on these platforms. Now I am assuming that a cache size of `1` maybe also physically has the same effect as if specifying no cache at all and that this is more of a "compatibility" value and we maybe should not allow it at all to preserve portability throughout the vendors.

The second problem is the `null` value for `$cacheSize` in Sequences. This tells the vendor to use it's own specific default cache size which differs on all platforms. The comparator will always mark a sequence as *changed* then when `null` was initially defined for the sequence and the sequence is introspected from database again. This will lead to repeated `ALTER SEQUENCE` statements in the schema tool which is not good. Also we cannot determine in the platform itself if the sequence has really changed as there currently is no `SequenceDiff` class which could get passed to `AbstractPlatform::getAlterSequenceSQL()`. Instead we only get the changed sequence object. I have put together some [examples](https://gist.github.com/deeky666/6c4e47a275b1c9e7c068) to clarify on what I mean as it is hard to explain.

So I'm asking for opinions and input on this :)
is `$cacheSize` guaranteed to be an unsigned `int` or `null`?