Merge pull request #188 from ctiller/bprintf

src/core/channel/call_op_string.c

src/core/channel/connected_channel.c

src/core/httpcli/format_request.c

src/core/support/string.c

src/core/support/string.h

src/core/surface/event_string.c

test/core/channel/metadata_buffer_test.c

test/core/end2end/cq_verifier.c

test/core/end2end/tests/census_simple_request.c

test/core/end2end/tests/simple_request.c

test/core/security/credentials_test.c

test/core/statistics/hash_table_test.c

test/core/transport/chttp2/hpack_table_test.c

test/core/transport/chttp2/stream_encoder_test.c

test/core/transport/chttp2/timeout_encoding_test.c

test/core/transport/metadata_test.c