update exchange, implement find for secondary index

abi for eosio.system contract, draft of unit-test, get_blockchain_parameters API function, asset switched to signed int64_t, fixes

Updated multi_index and new C DB API

Remove uint256 dependence from simpledb and abi_tests for now. Need to
later add support for key256 in ABI generation?

All tests except api_tests/transaction_test pass.

Dawn 417 restore release abi code generation

DAWN 646 - Fix problem where validation fails when creating new permission

DAWN-513 - Improve eosioc error message for 404 unknown endpoint

Tidy up http spacing

Makes error message more prominent

Fixes identity contract.
Only api_tests/transaction_tests fails.

grab max primary key in table on first call to available_primary_key
rather than on multi_index construction

Set minimum disk availability for install to 20 GB

sbrk fix for new wavm implementation

When wavm interface was refactored sbrk initialization was placed in the wrong spot. Due to shared memory, sbrk init needs to go after resetMemory() call

Merging master to avoid conflicts with PR #1496

Don't install fc

While technically fc is a standalone library, I don't consider eosio to be the distribution mechanism that one would obtain fc through. Plus, our fork of fc has some changes that may not be expected for general case users. Remove it from being installed