Use buffered writer

[bugadani]

Speed up flashing by infinite %!

Resolves #113

Flashing binary before:

WARN setting baud rate higher than 115200 can cause issues.
[00:00:07] ########################################      16/16      segment 0x1000
[00:00:00] ########################################       1/1       segment 0x8000
[00:06:01] ########################################     742/742     segment 0x30000

Flashing binary after:

WARN setting baud rate higher than 115200 can cause issues.
[00:00:00] ########################################      16/16      segment 0x1000
[00:00:00] ########################################       1/1       segment 0x8000
[00:00:25] ########################################     742/742     segment 0x30000

[jessebraham]

LGTM, thanks!