Allow the use of `this` without anything else.
Allow empty contract.
The @default clause must not contain @invariant.
Update API documentation.
[auto] Merge from Praspel

    • #a32f006 — Allow the use of `this` without anything else. (Ivan Enderlin, 2013-08-27)
    • #66666f8 — Allow empty contract. (Ivan Enderlin, 2013-08-27)
    • #174a7e4 — The @default clause must not contain @invariant. (Ivan Enderlin, 2013-08-27)
    • #98f5597 — Update API documentation. (Ivan Enderlin, 2013-08-29)

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Merge remote branch 'library-praspel/master'